Bridges for microelectromechanical structures

ABSTRACT

A conductive bridge in a second conductive layer may be utilized to join a pair of spaced apart conductive strips in a first conductive layer. A gap between the first and second strips may be bridged by the bridge while isolating both the first and second strips and the bridge itself from another conductor which extends through the gap between the first and second strips.

BACKGROUND

[0001] This invention relates generally to microelectromechanicalstructures (MEMS).

[0002] Microelectromechanical structures are physical structures whichmay be fabricated using microelectronic fabrication techniques. In thefabrication of MEMS devices, it is often desirable to isolate differentstructures electrically from one another. To this end, an air gap may bepositioned underneath an electrical connector. Such a structure may becalled a bridge since it allows an electrical connection over an air gapand provides for isolation from underlying devices.

[0003] For example, for multi-mode multi-band cell phone applications,an antenna switch multiplexer switches the antenna to a different modeor band, as well as between transmission and receiving. The multiplexerconsists of many individual switches. To route the signal lines, groundlines, and actuation control lines across each other, more that twometal layers are needed.

[0004] For example, an in-line cantilever beam metal contact seriesswitch generally requires two metal lines in order to allow theconnection. A first signal line may be in a first layer, a second signalline may also be in the first layer, an actuation element may be in thefirst layer, but the cantilever beam metal contact switch itself must bein at least a second layer.

[0005] Thus, there is a need for better ways to allow connections inMEMS devices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a top plan view of one embodiment of the presentinvention;

[0007]FIG. 2 is a top plan view of a second embodiment of the presentinvention;

[0008]FIG. 3 is a top plan view of a third embodiment of the presentinvention;

[0009]FIG. 4 is an enlarged cross-sectional view of a technique inaccordance with one embodiment of the present invention;

[0010]FIG. 5 is an enlarged cross-sectional view of the embodiment shownin FIG. 4 at a subsequent stage in accordance with one embodiment of thepresent invention;

[0011]FIG. 6 is an enlarged cross-sectional view at a subsequent stagein accordance with one embodiment of the present invention;

[0012]FIG. 7 is an enlarged cross-sectional view at a subsequent stagein accordance with one embodiment of the present invention;

[0013]FIG. 8 is an enlarged cross-sectional view at a subsequent stagein accordance with one embodiment of the present invention;

[0014]FIG. 9 is an enlarged cross-sectional view of a subsequent stagein accordance with one embodiment of the present invention;

[0015]FIG. 10 is an enlarged cross-sectional view of a subsequent stagein accordance with one embodiment of the present invention;

[0016]FIG. 11 is an enlarged cross-sectional view of a subsequent stagein accordance with one embodiment of the present invention;

[0017]FIG. 12 is an enlarged cross-sectional view of a stage subsequentto the stage depicted in FIG. 8 in accordance with another embodiment ofthe present invention; and

[0018]FIG. 13 is an enlarged cross-sectional view of a subsequent stageto that shown in FIG. 12 in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION

[0019] Referring to FIG. 1, a basic switch/transmission line co-planarwaveguide (CPW) 10 includes a control voltage line 18 that is routedunder a bridge 16 e across a ground line 12 a, including two stripsseparated by a gap 22, in accordance with one embodiment of the presentinvention. The ground lines 12 a, 12 b, and 12 c may be formed in afirst conductive layer. The signal line 16 may be made in a second,separate conductive layer. The control voltage line 18 may also be inthe first conductive layer.

[0020] The width of the CPW 10 generally scales with the width of thesignal line 16. The width of the signal line 16 may be reduced by usingboth of the first and second conductive layers in order to have thenecessary conductivity. Thus, the ground lines 12 may be made using athin bottom metal layer in one embodiment.

[0021] The width “W” of the bridge 16e may be small enough so that asacrificial layer (not shown in FIG. 1) underneath the bridge 16 e maybe removed during a release step. In one embodiment, the span of thebridge 16 e may be smaller than approximately five times the thicknessof the second, upper conductive layer so that the bridge 16 e is stiffenough not to collapse under voltage between the two conductive layers.

[0022]FIG. 2 shows a multiplexer 10 a which includes ground lines 12,signal lines 16 f and 16 g, as well as the signal lines 16 a through 16g in accordance with another embodiment of the present invention. Thebridge 20 a bridges the ground lines 12 c, the bridge 26 bridges theelements 16 f and 16 g, and the bridge 20 bridges the ground lines 12 a.Thus, the control voltage line 18 a may span all the way through threeseparate ground lines, 12 a, 12 b, and 12 c, to reach the ground line 12d.

[0023] Referring to FIG. 3, in accordance with still another embodimentof the present invention, the ground lines 12 a, 12 b, and 12 c may becrossed by control lines 18 b and 18 c. The control voltage line 18 bgoes under a bridge 34 and the control voltage line 18 c goes under abridge 35. The signal lines 32 and 36 are joined by the bridge 34. Thesignal lines 36 and 38 are joined by the bridge 35. By keeping the spanof each bridge 34 and 35 relatively small, multiple bridges may beneeded in some embodiments. Thus, the intermediate signal line portion36 may provide an island which allows the length of the bridges 34 and35 to be limited to the desired length.

[0024] In accordance with one embodiment of the present invention, abridge, such as a bridge 16 e, 16 c, 20, 26, 20 a, 16 c, 34, or 35, maybe formed by forming a dielectric layer 42 over a semiconductorsubstrate 40. The dielectric layer may be silicon dioxide or siliconnitride, as two examples.

[0025] Then, as shown in FIG. 5, a first or bottom conductive layer 12may be deposited on the dielectric layer 42 and patterned. Thepatterning of the layer 12 forms the central island 46 and the gaps 44.The bottom conductive layer 12 may be a composite of titanium, nickel,and gold, in one embodiment.

[0026] Referring to FIG. 6, the structure may then be covered with asacrificial layer 48. The sacrificial layer 48 may be deposited orspun-on in some embodiments. In one embodiment, the sacrificial layer 48may be made of polymeric materials, such as polyimide, resist, orflowable glasses, that reflow, shrink, melt, or vaporize at elevatedtemperatures.

[0027] Next, referring to FIG. 7, after lithography and etching, anchorholes 50 may be formed in the sacrificial layer 48.

[0028] A seed layer 52, for facilitating plating, may then be coatedover the structure shown in FIG. 7 to achieve the structure shown inFIG. 8. A thick resist 54 may be patterned as a mold for plating asshown in FIG. 9. Next, a bridge 16 may be plated, using the seed layer52 to facilitate adherence of the bridge 16, and using the resist 54 asa mold for defining the bridge 16. The second or top conductive layerforming the bridge 16 may be gold in one embodiment of the presentinvention.

[0029] After plating the bridge 16, the resist 54 may be removed. Theseed layer 52 may be etched away and the material 48 may be released,forming a void 58 under the bridge 16. In one embodiment, thesacrificial material 48 is released through the application of heat.

[0030] In another embodiment of the present invention, after thestructure shown in FIG. 8 is formed, etching may be used to form theU-shaped metal structure 52, as shown in FIG. 12. Instead of plating aseed layer, a heavier metal layer 52 may be formed in this embodiment.Thereafter, the air bridge may be formed by releasing the material 48,forming the void 60.

[0031] While the present invention has been described with respect to alimited number of embodiments, those skilled in the art will appreciatenumerous modifications and variations therefrom. It is intended that theappended claims cover all such modifications and variations as fallwithin the true spirit and scope of this present invention.

What is claimed is:
 1. A method comprising: forming a pair of conductivestrips in a first conductive layer, said strips separated by a gap;extending a conductor through said gap in the first conductive layer;and forming, in a second conductive layer, a conductive bridge thatelectrically couples said first and second strips in isolation from saidconductor.
 2. The method of claim 1 including forming a co-planarwaveguide.
 3. The method of claim 1 including forming a multiplexer. 4.The method of claim 1 including forming said second conductive layerover said first conductive layer.
 5. The method of claim 4 includingforming a release material under said conductive bridge and releasingsaid release material.
 6. The method of claim 5 including releasing saidrelease material by applying heat.
 7. The method of claim 1 includingforming said bridge with a span less than approximately five times thethickness of said second conductive layer.
 8. The method of claim 1including enabling a control voltage line to extend through a pluralityof ground lines formed by the same first conductive layer by providinggaps for said control voltage line to extend through said ground lines.9. The method of claim 8 including forming a plurality of bridges toconnect each ground line across each of said gaps.
 10. The method ofclaim 1 including depositing an insulator on a semiconductor structure,forming a release material on said insulator, and forming a seed layeron said release layer.
 11. The method of claim 10 including forming amold layer over said seed layer to define the shape of said bridge. 12.An integrated circuit comprising: a semiconductor structure; a firstconductive layer formed on said semiconductor structure, said firstconductive layer including a pair of conductive strips separated by agap, said first conductive layer further including a line extendingacross said gap in isolation from said strips; and a second conductivelayer over said first conductive layer, said second conductive layerincluding a bridge which couples said first strip to said second stripin electrical isolation from said line.
 13. The circuit of claim 12wherein said circuit includes a co-planar waveguide.
 14. The circuit ofclaim 12 wherein said circuit includes a multiplexer.
 15. The circuit ofclaim 12 wherein said bridge has a span less than approximately fivetimes the thickness of said second conductive layer.
 16. The circuit ofclaim 12 wherein said line is a control voltage line that extendscompletely through a plurality of ground lines, each of said groundlines including a gap allowing the passage of said control line, saidcircuit including a plurality of bridges that connect each of saidground lines across said gaps over said control voltage line.
 17. Amethod comprising: forming a first conductive layer including a pair ofconductive strips separated by a gap and a conductive line extendingthrough said gap between said strips; forming a second conductive layerincluding a bridge that electrically couples said first and secondstrips in isolation from said conductive line; and forming said bridgeover a release material that is removed in response to the applicationof heat.
 18. The method of claim 17 including forming a co-planarwaveguide.
 19. The method of claim 17 including forming a multiplexer.20. The method of claim 17 including forming said second conductivelayer over said first conductive layer.
 21. The method of claim 20including forming said bridge with a span less than approximately fivetimes the thickness of said second conductive layer.
 22. The method ofclaim 17 including enabling a control voltage line to extend through aplurality of ground lines formed in said first conductive layer byproviding gaps for said control voltage line to extend through saidground lines.
 23. The method of claim 22 including forming a pluralityof bridges to couple each ground line across each of said gaps.
 24. Themethod of claim 17 including depositing an insulator on a semiconductorstructure, forming a release layer on said insulator, and forming a seedlayer on said release layer.
 25. The method of claim 24 includingforming a mold layer over said seed layer to define the shape of saidbridge.
 26. An integrated circuit structure comprising: a semiconductorstructure; a first conductive layer formed on said semiconductorstructure, said first conductive layer including a pair of conductivestrips separated by a gap, said first conductive layer further includinga line extending through said gap in isolation from said strips; asecond conductive layer over said first conductive layer, said secondconductive layer including a bridge which couples said first strip tosaid second strip in electrical isolation from said line; and a releasematerial between said bridge and said line, said release material beingremovable in response to the application of heat.
 27. The structure ofclaim 26 wherein said bridge has a span of less than approximately fivetimes the thickness of said second conductive layer.
 28. The structureof claim 26 wherein said line is a control voltage line that extendscompletely through a plurality of ground lines, each of said groundlines including a gap along the passage of said control line, saidstructure including a plurality of bridges that connect to each of saidground lines across said gaps over said control voltage line.
 29. Thestructure of claim 26 wherein said release material is polymeric. 30.The structure of claim 26 including a conductive seed layer.